Species richness in the Arctic is low and decreases toward the north: there are about 1800 species of vascular plants, 4000 species of cryptogams, 75 species of terrestrial mammals, 240 species of terrestrial birds, 3000 species of fungi, 3300 species of insects [12], and thousands of prokaryotic species (bacteria and Archaea) whose diversity in the tundra has only recently started to be estimated.

In addition to the 235 plant species, 85 species of fungi, 74 species of ferns and other cryptogams (pteridophytes, psilophytes and lycopodiophytes) and 128 species of mosses and hepaticas (briophytes) have been identified, this latter group has exceptional diversity.

The vegetation of the Tristan da Cunha and Gough Island group is typical of southern cold-temperate oceanic islands, having relatively low species diversity, and a large prevalence of ferns and other cryptogams.

Thus we see that the flowering plant is essentially the equivalent of the asexual fern, and of the sporogonium of the moss, and the pollen cell and the embryo-sac represent the two spores of the higher cryptogams, and the pollen tube and the germinal vesicles and antipodal cells are all that remain of the sexual generation, seen in the moss as

The pollen cells are formed from mother cells by a process of cell division and subsequent setting free of the daughter cells or pollen cells by rejuvenescence, which is distinctly comparable with that of the formation of the microspores of Lycopodiaceæ, etc. The subsequent behavior of the pollen cell, its division and its fertilization of the germinal vesicle or oosphere, leave no doubt as to its analogy with the microspore of vascular cryptogams.

This description of the analogies of the pollen and embryo-sac of dicotyledons assumes that the general vegetative structure of this class of plants is equivalent to the asexual generation of the higher cryptogams.